First of all, I want to say that I am very new to this hobby (started just 3 months ago), so maybe I am talking complete nonsense and I appologise if I missed something,
BUT in my humble personal opinion you have a
pure chemical problem resulting from you, adding a big dose of
in order to increase your tank Ca level. In the Safety data sheet of this component from the
ESV site - we can read that it contains
100% Aqueous Solution of Non-hazardous inorganic salts - basically, you don't know what exactly you are putting in your tank and you don't know how it will react with the substances that are already in your tank. On top of that - you are adding only 1 of the 2-part system - maybe it's not working like it's supposed to, without the other part. Yes, you know you got your Ca high again (in a single day), but is that all? I think not. And we are seeing the results now.
As this
article shows, "Alkalinity in a marine aquarium is simply a measure of the amount of acid (H+) required to reduce the pH to about 4.2, where all carbonate and bicarbonate are converted into carbonic acid" and "when performing an alkalinity titration with a test kit, you are “counting” the number of bicarbonate ions present."
Basically, you messed up your tank chemistry and now you need to fix it. You already have proven that big water change with freshly mixed salt water, that you know has good parameters, makes your alkalinity drop. But you are wondering why it starts to creep back up after that? Well, you still have those "unknown" inorganic salts or their byproducts in your system, since you had done "only 30% wc". So, the chemical reactions are still continuing.
My suggestions is - you should do 1x 100% water change, then wait, measure, and after a day or couple of days do another 100% wc. Or when you notice an increase again, because maybe some of the inorganic salts are in your sand and rocks.
Before that, as suggested already, you should try to move your corals out of the tank for a few days. All this happening now is a lot of stress for them.
Just my 2 cents, which again - should be taken with a kinda huge grain of salt. GL an keep us updated!